To prevent global warming, there is strong demand for the reduction of the amount of CO2 discharged from automobiles. To reduce the amount of a CO2 gas emitted, it is mainly necessary to improve the fuel efficiency of automobiles. Technologies for improving fuel efficiency include fuel direct injection, increase in compression ratios, the reduction (downsizing) of engine weights and sizes by supercharging, increase in the boost pressure of turbochargers, etc. With these technologies introduced, fuel combustion tends to occur at higher temperatures and higher pressure in automobile engines, so that the temperatures of exhaust gases discharged from engines are elevated to nearly 1000° C., and that the temperatures of exhaust members such as exhaust manifolds, catalyst cases, turbine housings, etc. reach about 900° C. Exhaust members exposed to such high-temperature exhaust gases are required to have excellent heat resistance properties (oxidation resistance, high-temperature strength, thermal deformation resistance and thermal cracking resistance).
Exhaust members such as manifolds, etc. of automobiles used under severe conditions at high temperatures have conventionally been made of heat-resistant cast irons such as high-Si, spheroidal graphite cast iron, Ni-Resist cast iron (austenitic, cast Ni—Cr iron), etc., heat-resistant, ferritic cast steels, heat-resistant, austenitic cast steels, etc.
Among conventional heat-resistant cast irons and heat-resistant cast steels, ferritic, spheroidal graphite cast iron containing 4% Si and 0.5% Mo exhibits better heat resistance properties up to about 800° C., but poor durability at higher temperatures. Heat-resistant cast irons such as Ni-Resist cast iron, etc. containing large amounts of rare metals such as Ni, Cr, Co, etc., and heat-resistant, austenitic cast steels are used for exhaust members, because they meet both requirements of oxidation resistance and thermal cracking resistance at 800° C. or higher.
However, the Ni-Resist cast iron contains a large amount of expensive Ni, and has poor thermal cracking resistance because it has a large coefficient of linear expansion due to an austenitic matrix structure, and because its microstructure contains graphite acting as the starting points of fracture. The heat-resistant, austenitic cast steel has insufficient thermal cracking resistance at about 900° C. because of a large coefficient of linear expansion, though not containing graphite acting as the starting points of fracture. In addition, the heat-resistant, austenitic cast steel is expensive and thus has cost disadvantages because it contains large amounts of rare metals, and suffers unstable material supply affected by world economic situations.
From the aspect of economic feasibility, stable material supply and efficient use of global resources, it is desirable that heat-resistant materials used for exhaust members have necessary heat resistance properties with the minimum amounts of rare metals. Thus provided are inexpensive exhaust members, which enable the application of fuel-efficiency-improving technologies to popular cars, contributing to reducing the amount of a CO2 gas emitted. To reduce the amounts of rare metals contained as much as possible, the matrix structures of alloys are advantageously ferrite rather than austenite. In addition, because ferritic materials have smaller coefficients of linear expansion than those of austenitic materials, the ferritic materials have better thermal cracking resistance because of smaller thermal stress generated at the start and acceleration of engines.
However, general ferritic cast steels contain as little C as about 0.2% or less by mass, and do not contain melting-point-lowering alloying elements such as Ni, etc. unlike austenitic cast steels, having high melting points. Accordingly, general ferritic cast steels have low flowability of melts (hereinafter referred to as “melt flowability”), poor castability, so that they likely suffer casting defects such as misrun, cold shut, shrinkage cavity, etc. during casting. Particularly exhaust members having complicated and/or thin shapes do not have good melt flowability with a small C content, suffering casting defects such as misrun, cold shut, etc., resulting in a low production yield. Further, unlike the austenitic cast steels, the ferritic cast steels contain substantially no interstitial solute elements, easily subject to gas defects by hydrogen. Incidentally, the gas defects are defects generated by hydrogen contained in a melt, which does not keep dissolved not only in the melt (liquid phase) but also in a solid phase as the melt temperature lowers during casting, thereby leaving vacancies in the solidified castings.
To provide the improvement of castability, etc., the applicant proposed by JP 7-197209 A, a heat-resistant, ferritic cast steel having excellent castability, which has a composition comprising by weight C, 0.15-1.20%, C—Nb/8: 0.05-0.45%, Si: 2% or less, Mn: 2% or less, Cr: 16.0-25.0%, W and/or Mo: 1.0-5.0%, Nb: 0.40-6.0%, Ni: 0.1-2.0%, and N, 0.01-0.15%, the balance being Fe and inevitable impurities, and having an (α+carbide) phase (hereinafter referred to as “α′ phase”) transformed from a γ phase (austenite phase), in addition to a usual α phase (α ferrite phase), the area ratio of the α′ phase [α′/(α+α′)] being 20-70%. Because this heat-resistant, ferritic cast steel has excellent heat resistance properties at 900° C. or higher, it is suitable for exhaust members. Also, because it contains a large amount of C, it has good melt flowability, and thus improved castability.
In the heat-resistant, ferritic cast steel of JP 7-197209 A containing C in an amount more than consumed by forming NbC, carbide of Nb and C, C (austenitizing element) is dissolved in the matrix structure to form a solid solution, and forms a γ phase at high temperatures during solidification, the γ phase being transformed to an α′ phase during a cooling process to room temperature, thereby improving ductility and oxidation resistance. In an as-cast state, however, the γ phase is not transformed to the α′ phase sufficiently, but to martensite. The high-hardness martensite extremely deteriorates toughness and machinability at room temperature. To obtain good toughness and machinability, a heat treatment for precipitating the α′ phase while erasing martensite is necessary, but the heat treatment increases a production cost, providing economic disadvantages. The heat treatment also needs much energy, disadvantageous in the reduction of energy consumption.
As a cast member of ferritic, cast, stainless steel having a larger C content than those of general ferritic cast steels, JP 2007-254885 A discloses a thin casting member having improved high-temperature strength, which is made of ferritic, cast, stainless steel comprising C, 0.10-0.50% by mass, Si: 1.00-4.00% by mass, Mn: 0.10-3.00% by mass, Cr: 8.0-30.0% by mass, and Nb and/or V: 0.1-5.0% by mass in total, and has thin portions having thickness of 1-5 mm, a ferrite phase in the structure of thin portions having an average crystal grain size of 50-400 μm. In the cast member of JP 2007-254885 A made of ferritic, cast, stainless steel, thin portions of 5 mm or less are rapidly cooled after casting to reduce the average crystal grain size of the ferrite phase, thereby improving high-temperature yield strength, tensile strength and fracture elongation in thin portions.
However, in exhaust members having thick portions of 5 mm or more such as cylinder-head-mounting flanges, heat-insulation-plate-mounting bosses, bolt-fastening portions, thick converging portions, etc., the melt has a low cooling speed even in thin portions of 5 mm or less such as those near risers for preventing shrinkage cavities, and those adjacent to cavities where sand molds tend to be overheated. Such portions in the exhaust members have large average crystal grain sizes, resulting in low toughness (particularly room-temperature toughness). JP 2007-254885 A fails to disclose a measure for suppressing toughness decrease due to shape and thickness variations, casting designs, etc.
Also, the ferritic, cast, stainless steel of JP 2007-254885 A has improved melt flowability, which is obtained by lowering its melting point by containing Si in as large an amount as 1.00-4.00% by mass (about 2% or more in Examples), and improved high-temperature strength, oxidation resistance, carburizing resistance and machinability. However, this ferritic, cast, stainless steel has poor room-temperature toughness because it contains a large amount of Si dissolved in a ferritic matrix structure. Because Si dissolved in the ferritic matrix structure lowers the solid solution limit of hydrogen, a large amount of hydrogen is generated during solidification, accelerating the generation of gas defects.
Also, as a heat-resistant, ferritic cast steel having a larger C content than those of general ferritic cast steels, the applicant proposed by JP 11-61343 A, a heat-resistant, ferritic cast steel having excellent high-temperature strength (particularly creep rupture strength), which has a composition comprising by weight, C, 0.05-1.00%, Si: 2% or less, Mn: 2% or less, Cr: 16.0-25.0%, Nb: 4.0-20.0%, W and/or Mo: 1.0-5.0%, Ni: 0.1-2.0%, and N, 0.01-0.15%, the balance being Fe and inevitable impurities, and has a Laves phase (Fe2M) in addition to a usual α phase. Though this heat-resistant, ferritic cast steel has excellent high-temperature strength and good melt flowability, it has been found that it suffers the generation of gas defects extremely when it contains a large amount of Nb. Accordingly, this heat-resistant, ferritic cast steel has not been put into practical use for exhaust members so far.
As described above, because conventional heat-resistant, ferritic cast steels have poor toughness and machinability despite good melt flowability, and are likely to have gas defects, they are not necessarily suitable for exhaust members. The toughness and machinability can be improved by a heat treatment, but the heat treatment increases a production cost. Because gas defects cannot easily be removed, cast members with gas defects have to be discarded as defective products, resulting in a low production yield.